The invention is directed to a method of preparing the solution which remains after separation of the enzyme from the enzymatic resolution of racemates of an N-acetyl-DL-amino carboxylic acid in the presence of an L-amino acid acylase by electrodialysis.
When synthetically producing enantiomerically pure L-amino acids, the following indirect method is taken in many instances: The racemate which is obtained at first is acetylated and then the racemic N-acetyl-DL-amino acid is subjected to a resolution by an L-amino acid acylase. After separation of the enzyme, the L-amino acid freed by hydrolysis is isolated in a suitable manner. The nonhydrolyzed N-acetyl-D(L)-amino acid can be subjected again to a resolution of racemates after reracemization.
The solutions from the enzymatic resolution of racemates which remain after separation of the enzyme contain, for example, 0.15 to 0.7 mole/1 L-amino acid, 0.15 to 0.9 mole/1 N-acetyl derivates (primarily N-acetyl-D-amino acid and a lesser amount of N-acetyl-L-amino acid) and 0.15 to 0.7 mole/1 acetic acid, each of the last two acids in the form of an alkali metal salt, preferably a sodium salt, and slight amounts of effector salts, e.g. of CoCl.sub.2.
This solution is normally prepared by desalination by passing it over an ion exchange column filled with strongly acidic cation exchange material. Depending on the method of operation, one obtains a more or less desalinated amino acid solution, a strongly acidic fraction which contains the anions as free aids and, in addition, considerable amounts of used-up regeneration solutions which contain the exchanged cations and which must normally be removed as waste water. Additional problems occur in the case of poorly or only moderately soluble amino acids due to the fact that they can precipitate as free acids in the ion exchange column and can clog it. This is normally counteracted by elevating the temperature, which, in turn, causes disadvantages if thermally labile compounds are present.
German OS No. 29 07 450 teaches a method for preparing such solutions by electrodialysis. The known method is performed in an electrodialysis device which consists of a large number of chambers which are alternatingly separated from each other by cation and anion exchange membranes which, for their part, are positioned between a single electrode pair. Aside from the anode and the cathode cell, the chambers are operated in such a manner that one chamber funtions as feed and diluent chamber and the adjacent chamber as concentrate chamber in a constantly alternating manner. The solution to be prepared is circulated in the feed and diluent chambers and during the electrodialysis the anions and cations contained in the solution can pass through the anion exchange membrane and the cation exchange membrane into the adjacent concentrate chamber. The amphoteric amino acid is held in the feed and diluent chamber by virtue of the fact that the electrodialysis device is operated at a high current density near the limit current density. A pH barrier is built up thereby on the membranes which does not allow the amphoteric amino acid to pass through the membranes. It is possible to draw off enantiomerically pure amino acids in a good yield in the form of their aqueous solutions from the diluent chambers with the known method; however, it has the disadvantage that the anions and cations originally contained can remix and, as a consequence, only aqueous salt solutions can be drawn off from the concentrate chambers.